[docs] Update code snippet in docs for custom color palette

[ZeeshanTamboli]

• I have followed (at least) the PR section of the contributing guide.

Fixes #32926

contrastText is undefined when a new custom color is added in theme without a contrastText property. Contrast text color gets calculated for default palette primary, secondary etc. properties but not for custom color keys.

The component crashes if the contrast text is used within alpha due to this particular line where it accesses on undefined.

So we calculate the contrast text color with getContrastText before passing it to alpha when a custom color is provided in theme.palette.

Before: https://codesandbox.io/s/chippropscoloroverrides-failing-example-forked-m4nvsl?file=/src/App.tsx
Now: https://codesandbox.io/s/chippropscoloroverrides-failing-example-forked-1jlyfl?file=/src/App.tsx

Edit: The docs are updated instead.

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Docs preview: https://deploy-preview-32946--material-ui.netlify.app/material-ui/customization/palette/#providing-the-colors-directly

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against 15ee409

[siriwatknp]

👍 Looks good to me.

[yueveron]

Hi, I use jest reac test chip-component, it also show error "cannot read properties of undefined reading 'contrasttext'".
How can I fixed it in jest

[mnajdova]

Please open a separate issue with a reproduction.